1. Field of the Invention
The present invention relates to the field of radio technology. It relates in particular to an AM radio transmitter with an output power of at least 50 kW, comprising
(a) a low frequency part with a low frequency input and a modulation amplifier attached to the low frequency input; and PA1 (b) a high frequency part with an oscillator, a driver amplifier attached to the oscillator and a final stage tube in the form of a tetrode, which exhibits a plate, a cathode, a control grid and a screen grid, where the plate communicates with the output of the modulation amplifier and the control grid communicates with the output of the driver amplifier. PA1 (1) To the 250 kW short-wave transmitter, which is described in the aforementioned document and whose high frequency final stage is equipped with a high power tetrode of the BBC CQK 350-1 model. This tetrode operates with plate modulation in the class C operation with a d.c. plate voltage of 14 kV, a d.c. screen grid voltage of 1300 V and a d.c. control grid voltage of -900 V and has an efficiency of 85.2% (Brown Boveri Mitt. 69, 1982 (6), p. 219). PA1 (2) To the 100 kW short-wave transmitter, which is described in the document Brown Boveri Mitt. 67, 1980 (3), pp. 215-219 and whose high frequency final stage is equipped with a high power tetrode of the BBC CQK 50-2 model. This tetrode operates with plate modulation in the class C operation with a d.c. plate voltage of 11 kV, a d.c. screen grid voltage of 800 V and a d.c. control grid voltage of -600 V (BBC Brief Information Catalog Electronic Tubes, Document no. CH-E 3.30475.8 D/F/E/8 of 1982/1983). PA1 (c) the final stage tube is a low voltage tetrode, which is operated at a d.c. plate voltage &lt;10 kV; and d) the final stage tube exhibits a plate efficiency greater than 80%. PA1 (a) the cathode of the final stage tube is designed as an indirectly heated, BaO-containing matrix cathode; PA1 (b) the control grid and the screen grid of the final stage tube are spaced less than 1 mm apart (a2, a1) and from the matrix cathode; PA1 (c) the control grid is driven with a control grid voltage of greater than -250 V; and PA1 (d) the screen grid is driven with a screen grid voltage of less than 650 V. PA1 (a) the modulation amplifier is designed as a digital pulse step modulation (PSM) amplifier; PA1 (b) an A/D converter is arranged between the modulation amplifier and the low frequency input; where PA1 (c) the modulation amplifier comprises a plurality of identical switching stages; and PA1 (d) the number of switching stages is less than 20, preferably less than 14.
Such a radio transmitter is known, e.g. from the document Brown Boveri Mitt. 69, 1982 (6), pp. 212-217.
2. Discussion of the Background
The existing kinds of radio transmitters with amplitude modulation (AM) are used especially in the short-wave range (about 3.9-26.1 MHz). The low frequency portion provides for the processing and power amplification of the low frequency signal that is to be transmitted and which is given then (during the typical plate modulation) on the plate of the final step tube. The carrier frequency oscillator with the subsequent driver step makes available in the high frequency portion a power amplified carrier signal, which gets on the control grid of the final stage tube and together with the plate voltage oscillating in synchronous operation with the low frequency signal delivers the desired AM signal to the load, the aerial.
Since such radio transmitters operate usually in a power range of more than 50 kW to a few 100 kW output power, the efficiency, i.e. the ratio of the input power to the usable power, plays a central role in the development and design of such a transmitter. The final stage tube has the controlling portion of the total efficiency of the transmitter, which can be greater than 70%.
Its efficiency, the so-called plate efficiency, is, among other things, proportional to the expression 1-(U.sub.S /U.sub.AO), where U.sub.S (also called U.sub.AR) denotes the non-modulatable residual voltage and U.sub.AO (also called U.sub.A) the d.c. plate voltage. Therefore, the plate efficiency increases with constant residual voltage with increasing d.c. plate voltage (Brown Boveri Mitt. 71, 1984 (5), p. 199).
Therefore, a good plate efficiency requires in all cases a high d.c. plate voltage U.sub.AO, so that the non-modulatable residue U.sub.S remains relatively small in comparison (see to this end: Meinke/Bundlach, Handbook of High Frequency Technology, 3rd ed., Springer-Verlag 1968, pp. 1035-1037). Therefore, the usual operating voltages for high power transmitters with high power tetrodes in the high frequency final stage ranges today from 10 to 14 kV (see to this end: Meinke/Gundlach, Handbook of High Frequency Technology, 4th ed., Springer-Verlag 1986, p. p9).
As examples of these values obtained in the state of art reference is made to the following two short-wave transmitters.
The comparatively high plate voltages require in connection with the plate modulation suitably designed modulation amplifiers that have to deliver at a d.c. plate voltage of 14 KV output voltages ranging from 0 to 28 kV.
If, for example, a pulse step modulator PSM, i.e. a digital switching amplifier, is used as the modulation amplifier, 32 switching stages, whose output voltages are added to the desired plate voltage, are required, for example, within this PSM (Brown Boveri Tech. 74, 1987 (6), pp. 296-302). Since every single one of these 32 high power switching stages demands suitable space, separate cubicles must be provided for the PSM in the transmitter.
However, the high d.c. plate voltage and the dielectric strength necessary for this voltage for other components of the transmitting circuit also lead to increased space requirements, so that the transmitter must have in total a very complex design.
One possible decrease in the d.c. plate voltage would lead in the case of the conventional final tubes to a decrease in the design complexity, but would reduce, on the other hand, the efficiency of the tube and thus the total efficiency of the transmitter to a nonjustifiable value.